Genistein and daidzein are estrogenic isoflavones found in soybeans. Although they are much less potent than estradiol, infants fed soy formula have serum concentrations that are up to 4 orders of magnitude higher than estradiol. Anatomic, sonographic, and other biochemical evidence of estrogen exposure has not been looked for in infants fed soy. Whether such widely used products do or do not have clinically detectable estrogenic activity in infants, the group with the highest exposure per kilo, is important clinically;in addition, it may be a crucial test of the ability of laboratory measures of estrogenicity to predict actual endocrine response in humans. We have done the field work for pilot studies to examine the natural history of estrogen responsive physiology in the newborn, and have used the results to develop a longitudinal study of the possible estrogenic effects of soy formula on infants. Enrollment in this study began in August, 2010 at Children's Hospital of Philadelphia. Using samples from this project, we collaborated with the Centers for Disease Control to show that, in iodine deficient infants, higher perchlorate exposure is associated with higher thyroid stimulating hormone level but not lower T4. This finding needs further research, which we will do in our new longitudinal study.